Thru-feed centerless grinding is a manufacturing process commonly used to generate a precise cylindrical geometry on a workpiece. Since thru-feed centerless grinding can generate the cylindrical form with tight tolerances while forming a fine surface finish, it has become very popular in the bearing, automotive and fuel system industries for the production of precision bearing races, shafts, plungers, etc. Thru-feed centerless grinding is especially widely used in fuel systems manufacturing to generate a sub-micron precision, cylindrical shape match diameter, for example, on a fuel injector plunger or needle valve element.
A thru-feed centerless grinding machine includes three primary components: a grinding wheel, a work-rest blade and a regulating wheel. Grinding of a workpiece takes place between the rotating regulating wheel and the rotating grinding wheel. The work-rest blade is used to support the workpiece at a given center-height. The regulating wheel, usually formed of a rubber-like material having a high coefficient of friction, moves the workpiece axially through the machine while rotating the workpiece at a constant speed to during grinding. The regulating wheel is tilted at a small angle to generate a force component in the axial direction to push the workpiece through the gap between the regulating and grinding wheels.
The precision tolerance requirements for diesel fuel system components have steadily been tightened in recent years to improve the performance and durability of diesel fuel system components. Truing the regulating wheel to the proper form has been identified by the practical grinding engineers and researchers as the most critical step in the set-up of the precision thru-feed centerless grinding process. In the grinding arts, a "truing" operation is performed on a regulating wheel in order to insure that the profile of its peripheral surface is cut to a proper shape. By contrast, a "dressing" operation creates the desired frictional condition on the surface of the regulating wheel. Truing and dressing operations are often performed on both newly manufactured and used regulating wheels to initiate and maintain a desired profile and proper surface conditions on the wheel. The truing operation properly shapes the wheel by grinding away a portion of the peripheral surface of the wheel in accordance with a pattern. In many conventional thru-feed centerless grinding machines, the regulating wheel is trued and dressed by a single-point diamond tool. The single point diamond is traversed along a straight line at an angle relative to the regulating wheel axis of rotation. However, the workpiece will experience a change in the size of the gap between the grinding and regulating wheels as the workpiece moves along the gap due to the tilted position of the regulating wheel. This change in the size of the gap results in interference between the outer surface of the workpiece and the outer surface of the regulating wheel. This interference is a major source of error in conventional sub-micron precision thru-feed centerless grinding.
Moreover, accurate and time consuming set-up of the grinding machine is required to achieve sub-micron precision results. It is common for a manufacturing engineer to spend months, sometimes years, of trial-and-error experiments to set-up the machine and develop a capable sub-micron precision grinding process. Due to the difficulty, time and expense involved in the set-up process, the set-up in a production centerless grinder rarely changes once the process is set. This strategy greatly hinders the flexibility in the use of the grinding machine and consequently increases capital expenditures in the precision grinding operations.
U.S. Pat. No. 4,004,568 to Maxey discloses a method and apparatus for dressing regulating wheels in a centerless grinding machine that includes a cylindrical dressing wheel positioned to peripherally engage the regulating wheel. However, the dressing wheel is positioned on the same side of the regulating wheel as a workpiece would be positioned for grinding. Moreover, this reference does not discuss the significance of the diameter of the dressing wheel. Therefore, this grinding machine inherently produces undesirable interference between the outer surface of the workpiece and the outer surface of the regulating wheel thereby making sub-micron precision grinding practically impossible.
U.S. Pat. No. 3,534,502 to Lovely discloses a grinding wheel apparatus including a dressing tool wherein a work-piece diameter and the dressing tool diameter are identical and are equidistant from the grinding wheel center. The dressing tool, however, is used to dress the grinding wheel, not a regulating wheel. In fact, this grinding device is not a thru-feed centerless machine and therefore does not suggest using a regulating wheel.
U.S. Pat. No. 1,860,614 to Hohnhorst et al. discloses a mechanism for truing and dressing an auxiliary grinding wheel of a centerless grinder including an abrading wheel. However, the abrading wheel is positioned at a different height, relative to the center of the auxiliary grinding wheel, than the workpiece. Moreover, the abrading wheel may have a diameter different than the finished workpiece. Thus, this grinding machine inherently produces undesirable interference between the outer surface of the workpiece and the outer surface of the regulating wheel thereby making sub-micron precision grinding practically impossible.
U.S. Pat. No. 4,570,386 to Unno et al. discloses a regulating wheel dressing system for achieving precision grinding by dressing the regulating wheel using a single point truing and dressing tool and varying the position of the single point truing and dressing tool in the X direction. However, the movement path of the single point truing and dressing tool is determined mathematically and therefore it is difficult to determine the exact line for a given set of conditions. More importantly, it is practically impossible to control the movement of the single point truing and dressing tool so as to ensure the tool moves along the mathematically defined path. In addition, the mathematically defined path changes as the size of the regulating wheel changes, for instance, after each truing operation. As a result, no practical embodiment of this type of truing and dressing assembly has been produced which is capable of achieving sub-micron precision grinding.
Consequently, there is a need for a thru-feed centerless grinding machine having a truing and dressing system capable of optimally truing and dressing a regulating wheel to ensure sub-micron precision grinding.